1. Technical Field
Embodiments of the present disclosure relate to an exhaust emission control system of an internal combustion engine.
2. Description of Related Art
In an internal combustion engine, a filter may be provided in an exhaust passage, for curbing release of particulate matter (which will be called “PM”) contained in exhaust gas to the outside. Since the PM in the exhaust gas can be trapped by and gradually deposited in the filter while the engine is operating, a filter regeneration process may be performed so as to prevent clogging of the filter. In a diesel engine, for example, an air-fuel ratio of exhaust gas may be generally kept on the lean side; therefore, unburned fuel can be supplied to the exhaust gas, to be oxidized by an oxidation catalyst, or the like, provided in the exhaust passage. This may increase the exhaust temperature, and oxidize and remove the deposited PM.
Generally, the filter can have a main body portion that extends along flow of exhaust gas, and the PM in the exhaust gas may be trapped in the main body portion. However, the state of deposition of the PM in the filter is not always uniform; the PM deposition amount may vary among location regions of the filter, depending on the temperature distribution in the filter caused by flow of exhaust gas, changes in the load of the engine with time, and so forth. The variations in the PM deposition amount among local regions of the filter may cause an excessive rise in the temperature of the filter during the filter regeneration process, which may undesirably result in deterioration of the filter, for example. Thus, according to a technology described in Japanese Patent Application Publication No. 2011-137445 (JP 2011-137445 A), two or more sets of electromagnetic-wave transmitting and receiving means can be arranged in a direction of exhaust flow in the filter, and spatial distribution (variations) of the PM deposition amount in the filter can be measured by using detection results of the above means.